1. Field of the Invention
This invention relates generally to the chemical vapor deposition of tungsten in the fabrication of microelectronic devices, and more specifically to methods for controlling the grain size of the tungsten films.
2. Description of the Related Art
Tungsten is widely used in microelectronic devices to provide electrical connection between device elements. Tungsten films are deposited on semiconductor substrates to fill contact and via holes and to form interconnect lines. In device fabrication, chemical vapor deposition (CVD) is used to form tungsten films through the reduction of tungsten hexafluoride (WF6) by hydrogen (H2) or silage (SiH4). It is common for the semiconductor substrate to have device features already patterned onto the substrate before the tungsten is deposited. To prevent unwanted deterioration of these features, and to prevent diffusion of the tungsten into the substrate, the tungsten layer is typically deposited using low temperatures, generally below 400xc2x0 C.
Because tungsten does not bond well to silicon or silica based insulating layers, a thin adhesion layer of titanium or titanium nitride is often first deposited onto the substrate to improve tungsten adhesion. Tungsten is typically deposited onto the adhesion layer using two or more discrete stages, as described in U.S. Pat. No. 5,795,842 to Hancock, U.S. Pat. No. 5,407,698 to Emesh, and U.S. Pat. No. 5,956,609 to Lee et al. In a first stage, a thin nucleation layer, also called the glue layer, is deposited, usually by the reduction of tungsten hexafluoride by silage. The nucleation layer further improves the adhesion of tungsten to the substrate surface. After deposition of the nucleation layer is complete, bulk layers of tungsten are subsequently deposited over the nucleation layer to grow the tungsten film to the desired thickness. Typically, the bulk layers are formed by reduction of tungsten hexafluoride by hydrogen.
As device dimensions shrink, so do the size and aspect ratios of the device features that the tungsten film must cover. The challenge is to fill these small features with tungsten completely, without leaving voids or seams in the tungsten layer, which can diminish device performance.
As illustrated by comparison of FIG. 1A to FIG. 1B, the minimum dimension of a gap 100 that can be filled without voids 102 is determined by the size of the grains 110 of tungsten that form the film. In FIG. 1B, the smaller grains 110 result in fewer and smaller voids 102. The grains 110 are crystals of tungsten that grow during the deposition process, and are typically shaped as columns that grow in the direction of the crystal. Voids 102 and seams form when two or more individual crystals grow until their boundaries reach each other without filling all of the space in between them. Hence, by reducing the tungsten film grain size, smaller features can be filled.
Previous methods for decreasing tungsten grain size have included using a low WF6 flow rate, using a low deposition temperature (400xc2x0 C. or below), and using a high chamber pressure (for example, approximately 100 Torr).
A process is provided for depositing a tungsten film having a small grain size on a semiconductor substrate. The process is suitable for use in filling contact and via holes on the substrate which are very small and have high aspect ratios while avoiding the problem of voids and seams. The process is also suitable for depositing smooth tungsten films to be patterned into interconnect lines.
In the embodiments of the invention, the nucleation layer is deposited such that the nuclei clusters that form the layer are small and have a high density, to provide high surface coverage of the substrate surface with few vacancies. The subsequently deposited bulk tungsten layer has a reduced grain size.
In one example, a tungsten film having a mean grain size of less than 145 nm can be deposited on a substrate by depositing a nucleation layer with a mean nuclei cluster size of less than 100 nm and depositing bulk layers over the nucleation layer. The nucleation layer controls the grain size of the tungsten film.
Various process parameters are used to deposit a nucleation layer having small, densely packed nuclei that results in a tungsten film having a small grain size.
In one method, the temperature of the substrate is raised to over 400xc2x0 C. for deposition of the nucleation layer, while deposition of the subsequent bulk layer is accomplished at substrate temperatures below 400xc2x0 C.
In one method, the thickness of the nucleation layer is controlled to be no more than approximately 600 xc3x85 thick, typically between 150 xc3x85 and 530 xc3x85 thick.
In one method, the flow rate of SiH4 is between 35 sccm and 80 sccm, typically 45 sccm, during the nucleation layer deposition, so that the ratio of WF6:SiH4 flow rates during deposition of the nucleation layer is in the range of 1:1 to 17:1, typically 9:1.
In one method, the pressure of the deposition chamber is maintained at between 40 and 80 Torr, typically 60 Torr for deposition of the nucleation layer. The subsequent bulk layer is deposited at a pressure of 40 Torr.